It's the sort of delightfully innocent exchange that plays out between young siblings every day.
But four-year-old Delilah O'Donoghues earnest lecture on behaviour to her two-year-old brother, Gabriel, could be about to make their family a fortune.
Last night their father, Lee, told how he had initially posted a 65-second video clip of his children on YouTube thinking simply that friends and family could find it amusing.
Within days it had become an internet sensation, attracting more than half a million viewers as well as requests for interviews from media outlets here and in America.
Mr O'Donoghue and wife Alex have described the interest in the video as surreal.
The 42-year-old said he began filming on his smartphone about two minutes into the little girls scolding.
"I watched from afar until I realised he must be really getting it from her," he said.
"In classic Delilah style she decides to give it to him straight."
Mrs O'Donoghue, 40, a teacher, said of their daughter: "Delilah's very bossy - it's how little girls are, I think."
"She has scolded Gabriel in the past. But it is as much about trying to teach him. They are good little buddies really."
The clip shows Gabriel receiving the dressing down from his sister after he found himself in hot water with their father for spitting at another child in the playground.
Delilah was quickly on hand with sisterly advice - some of which belies her young years.
"I'll keep remembering you being about two and I'll tell my friends that too," she tells him.
"You're nearly three, you should toughen up a bit."
As the lecture continues, Gabriel hangs his head, swings his legs, fidgets with his hands and looks away.
She tells him: "When Mum and Dad said you don't do that, you don't do that... and you don't spit."
Referring to the child with whom Gabriel had apparently had the playground spat, she adds: "That boy's old enough. You're only two."
"You're not old enough for that boy to... do a fight."
Delilah completes her reprimand with the words: "Think about it, Gabriel. Think about it."
Mr O'Donoghue, a company director, said he posted the clip, titled You Need to Toughen Up A Bit, on YouTube last week.
He said: "I'm not the sort of father who follows my children around with a digital camcorder. I just spotted it and thought it was comedy gold."
He and his wife have now signed up with global video marketing company Viral Spiral, which represents hundreds of home-movie makers with the aim of maximising profits from clips posted online.
The couple said they had not decided how to proceed with marketing linked to the video, saying their first concern is the impact any deal would have on their children.
The most viewed viral video of all time is of another pair of British siblings.
The clip, called Charlie Bit My Finger, shows a little boy having his finger bitten by his baby brother and has been viewed more than 480 million times since it was posted in 2007.
Their parents, Howard and Shelley Davies-Carr, also represented by Viral Spiral, have earned about £100,000 from advertising revenue and marketing deals. By yesterday, 593,000 YouTube viewers had clicked on Delilah and Gabriel's video.